The present invention relates to a polytetrafluoroethylene (PTFE) tube and more particularly to a PTFE tube for a flexible hose. In particular the invention relates to a PTFE tube having a smooth bore for use in the production of a lined hose assembly further comprising hose braids, external hose protection and end fittings.
It should be noted that there are two basic types of internal tube configuration;
smooth bore tubes, as their name suggests, have a substantially convolution free internal surface;
in contrast, internally convoluted tubes, as their name suggests, comprise a number of distinct peaks and roots.
Of course smooth bore tubes are often not totally devoid of bumps and indentations and may show rippling. This is however in sharp contrast to the induced peaks and roots of an internally convoluted tube.
PTFE is a unique material and is favoured for applications in the transport of foodstuffs and chemicals because of its chemical resistance and non-stick nature. However PTFE is not naturally elastic.
Producing a flexible PTFE tube for certain applications, particularly high pressure applications, where fluids, more particularly gases and vapours, are pumped through the tube has proved difficult. Indeed, it had previously been thought that many convoluted PTFE tubes would not be suitable for such applications because the xe2x80x9cthinningxe2x80x9d of the walls to produce xe2x80x9cflexxe2x80x9d was expected to result in increased permeation to fluids.
To reduce permeation one or more of the following techniques have hitherto been employed:
1. Wall thicknesses have been increased;
2. Higher grade polymers have been used; or
3. Polymers have been processed to have increased crystalinity.
Increasing the wall thickness decreases the flexibility of the finished product as well as increasing its weight and cost.
Increasing crystalinity increases the flexural modulus of the material thus decreasing the flexibility and this also incurs a reduction in flex life.
Du Pont, for example, define crystalinity as being low (50%), moderately high (72%) or very high (82%). At low crystalinity the product has a flexural modulus of 54,000 psi and a relative permeability to CO2 gas of 6; at moderately high crystalinity the product has a flexural modulus of 150,000 psi and relative permeability to CO2 gas of 0.8 and at a very high crystalinity the product has a flexural modulus of 170,000 psi and a relative permeability to CO2 gas of 0.2.
Most corrugated products are made by a process which convolutes or concertinas the product and have walls which are substantially uniform in thickness throughout. Typical processes include those described in GB 1543586 and GB 2293222.
EP 474449 B1 on the other hand discloses a corrugated plastics tube which has been subject to a compression force to displace material in the root region. It is characterised in that the compression force applied was sufficient to take the plastics of the tube, which was at a temperature below its melt temperature, beyond its elastic point. This can be achieved at any temperature below the melt temperature and the patent makes no specific teaching in this regard. Furthermore, the patent relates to plastics in general and is directed to producing flexibility. It is not particular to PTFE (although PTFE is specified) and it does not address the problem of producing tubes with improved permeability resistance to gases.
In contrast the present invention, which is particular to PTFE, teaches that a novel product is obtained by a process comprising
1. subjecting the PTFE tube to a deformation force at a temperature at or above the gel transition temperature of PTFE to produce constrained convolutions having a thinned wall W1; and
2. cooling the PTFE tube to below the gel transition temperature whilst continuing to constrain the deformations having the thinned wall W1 until the convolutions having the thinned wall W1 have become stable.
This product is characterised in that the convoluted PFTE tube has an improved resistance, of greater than 7.6%, to permeation by comparison with the non-convoluted tube, the comparison being made between tubes of (i) equal nominal bore ID; and (ii) equal weight of PTFE per unit length.
This improved resistance to permeation is indicative of the fact the product processed in this manner has a different form to one not so processed. This can be confirmed by way of the test procedure set out in the specific description.
Surprisingly, the applicants have discovered that by processing the PTFE, which term includes modified PTFE, in a particular manner they are able to reduce permeation rates for a given thickness of PTFE. That the PTFE processed in this manner has a changed form can be characterised by amongst other things, its improved resistance to permeation and increased tensile strength.
According to a first aspect of the present invention there is provided a PTFE tube comprising external roots and peaks which tube is obtainable from a non-convoluted tube having an original wall thickness W0 and an internal diameter ID by a process in which a region of the tube is thinned to provide external convolutions with a root wall thickness W1, characterised in that the convoluted PTFE tube has an improved resistance to permeation of greater than 7.6% by comparison with the non-convoluted tube, the comparison being made between tubes of (i) equal nominal bore ID; and (ii) equal weight of PTFE per unit length.
Preferably the PTFE tube has a smooth internal bore.
In one embodiment the smoothbore has a rippled appearance.
According to a further aspect of the present invention there is provided a method of producing a PTFE tube comprising external roots and peaks from a non-convoluted tube having an original wall thickness W0 comprising:
1. subjecting the PTFE tube to a deformation force at a temperature at or above the gel transition temperature of PTFE to produce constrained convolutions having a thinned wall W1; and
2. cooling the PTFE tube to below the gel transition temperature whilst continuing to constrain the deformations having the thinned wall W1 until the convolutions having the thinned wall W1 have become stable.
Preferably W1 is less than 25% of W0.
More preferably W1 is about 20% of W0. In a preferred embodiment the PRFE tube is produced on a mandrel of substantially the same size as the internal diameter of a plane cylindrical PTFE paste extruded tube such that the resulting tube is a smoothbore, externally convoluted, tube. The resulting smoothbore tube has a rippled appearance.
That the deformation has become stable can be characterised by an increase in tensile strength indicating that the deformation is a xe2x80x9cyieldxe2x80x9d deformation. The deformation can be further characterised in that it is reversible. i.e. when the deformed material is reheated to at or above the gel transition temperature without a restraining force in place, it returns substantially to its original form.
It is also possible to determine whether or not the PTFE was deformed at a temperature above or below the gel transition temperature. A tube deformed below the gel transition temperature will revert partially or substantially to its original form at temperatures below the gel transition temperature whereas one deformed at or above the gel transition temperature will only revert substantially to its original form at or above the gel transition temperature.
The increase in tensile strength can be seen by conducting a simple test. A longitudinal section is taken from a convoluted tube prepared in accordance with the invention and is gripped on either side of a root. It is then pulled apart until the section breaks at the root. By first determining the thickness and width of the root and noting the force applied to break the tube at its root the breaking force per cross sectional area can be calculated. Another section of the tube is then heated to above the gel transition temperature so it reverts to its starting conformation and the section is then subjected to the same test i.e. it is pulled along the longitudinal axis of the tube. Typical results obtained will be 41368 kPa (6000 psi) for a plain tube and 75842 kPa (11000 Psi) for a convoluted tube manufactured in accordance with the invention.
The permeability properties of PTFE tubes deformed in this manner were totally unexpected as a product which was more permeable was expected as a consequence of a xe2x80x9cthinningxe2x80x9d of the walls.
For the avoidance of doubt the term gel transition temperature as used herein refers to the temperature at which PTFE becomes more transparent and amorphous. This is at a temperature of between 325xc2x0 C. and 340xc2x0 C. and is generally considered to be at a temperature of 327xc2x0 C. This temperature is sometimes inappropriately, in a processing context, referred to as the melt temperature, see for example D. I. McCaine xe2x80x9cCo-polymers with hexafluoropropylenexe2x80x9d see page 630. The true xe2x80x9cmeltxe2x80x9d temperature is the temperature at which the polymer melts from its gel state to form a liquid at which point it also begins to degrade and evaporate rapidly. This is at a temperature of above 550xc2x0, approaching xe2x80x9cred heatxe2x80x9d, see for example R. J. Plunkett the inventor of PTFE.
Without wishing to be bound by theory it is believed that at temperatures above 327xc2x0 C. a given applied deformation force is less likely to cause xe2x80x9ccutxe2x80x9d than the same deformation applied at temperatures below 327xc2x0 C. Furthermore because the material is elastically deformed as opposed to being xe2x80x9ccutxe2x80x9d it benefits from improved characteristics, for example, improved resistance to permeability and increased tensile strength. These characteristics show themselves in the convoluted tubes ability to revert substantially to its original form on re-heating to above 327xc2x0 C. without a restraining force in place. The greater the xe2x80x9ccuttingxe2x80x9d during processing the greater the depth of any xe2x80x9cnicksxe2x80x9d which appear in the so reverted product and the less it will resemble its original form.
At processing temperatures below 327xc2x0 C. the deformations will include, for a critical force, deformations beyond the products elongation break point which will not repair. Only deformations beyond yield, and not those beyond the products elongational break point will revert to their original shape on re-heating to above 327xc2x0 C. xe2x80x9cCuttingxe2x80x9d can, of course, also occur at temperatures above the gel transition temperature if the deformation caused by the force is sufficient. The critical deformation will, however, be less at a temperature of below 327xc2x0 C. For example, a smooth bore convoluted tube processed at below the gel transition temperature will, above a critical deformation, exhibit significant cut. Below this critical deformation a tube can only be thinned in the root region to between one third to one quarter of its original thickness. When processing at temperatures above the gel transition temperature, the tube can be thinned to about one fifth of its original thickness without exhibiting cutting.
Thus, according to another aspect of the present invention there is provided a PTFE tube comprising external roots and peaks, which tube is obtainable from a non-convoluted tube having an original wall thickness W0 by a process in which a region of the tube is thinned to provide external convolutions with a root wall thickness W1 characterised in that W1 is less than 25% of W0.
Preferably W1 is about 20% of W0.
The term xe2x80x9creturns substantially to its original formxe2x80x9d is intended to mean that the reverted tube does not have significant convolutions, although it may show signs of limited damage caused by deformations beyond elongation at break point in the form of cuts or nicks. The product will, however, return to within 20%, more preferably 10% and more preferably still 5% of its original wall thickness W0.
Because the force applied to the tube to form roots is 3-dimensional it cannot readily be determined. However, the deformation can be measured as indicated above. As a general rule greater deformations can be achieved without cutting at higher temperatures. Above 327xc2x0 C. deformation without cutting is about 20% better than below 327xc2x0 C. as indicated by the greater thinning which can be achieved when processing at temperatures above the gel transition temperature. Of course the deformations can not be fixed above 327xc2x0 C. therefore to fix the deformations a restraining force needs to be maintained whilst the temperature is dropped to below 327xc2x0 C. such that the deformations become stable.